Add oslo.rootwrap to global requirements

Per-project rootwraps (cinder-rootwrap, nova-rootwrap, neutron-rootwrap)
are being replaced by the common oslo-rootwrap (oslo.rootwrap package).

As a first step in the migration, oslo.rootwrap needs to get installed
as a runtime dependency.

Change-Id: I90d5ca8504855edac8a09492203100ba7f708d80